Hopefully this gets in right topic. I used BLAST comparisons semi randomly and found that opioid receptors were at least 20% identical with cannabinoid, thrombin and fatty acid receptors.I don't know if that is new discovery or not. BLAST program can find connections that humans may have not noticed. (http://www.sidesofsentience.com/2012/07 ... -gene.html)

Some outtakes:

Blood vessels were regulated by some of these receptors. Bradykinin widens blood vessels lowering blood pressure and angiotensin has opposite effects to bradykinin. Thrombin and platelet activating factor receptor cause blood coagulation. Possible that during some ancient era these receptors were more similar to FFAR and they all are probably able to lose sensitivity if something over activates them. For example if blood gets fatty it make it more viscose and likelier to spread slower but if FFAR senses too much fat it could widen blood vessels or block blood coagulation to minimize viscosity. For example omega-3 polyunsaturated acid inhibits blood coagulation. Another free fatty acid also widened blood vessel in heart.

Uncaring pleasure that fatty food gives (most likely animal product like meat, milk, cheese etc) motivates animals to keep eating other animals or risk hunger and all the emotional suffering it gives. This enjoyment may also make animals willing to keep on eating cute little animals even if they suspect they suffer when they wiggle in the mouth.

Similar uncaring attitude applies to almost any meaty food. I know well that sausage is greasy sliced up animal leftover stuck in intestines but if i eat it i still get pleasant warm buzz through my body and mind that makes me happy i ate it. As extra experiment during the day i wrote this i drank about 1 gram of sunflower oil. I drank i slowly and cautiously but within seconds i got intense and pleasant calm. Contact with tongue seemed enough to start this feeling and it lasted strongly for about 5 minutes. Possible that naturally oily skin in animals is enough to start predatory eating frenzy by giving very pleasant mood.

Dairy products are also very fatty and they feel good even if people know this lactose could give them stomach pains, nausea and diarrhea.

Fat is quite addictive which becomes obvious if you skip fatty foods for even 1 day. By afternoon this could reach the point of being tired, grumpy, restless and angry over many things but these symptoms disappear after greasy food.

This addiction is probably reason why people willingly eat themselves fat knowing well that they will shorten their life, cause joint pains, become worse looking, cause deaths of more animals in slaughterhouses, lose attractiveness and more likely to stay single/lonely but still they keep on eating like heroin addicts willing to risk HIV infections through needle sharing or prostitution.

Mu-opioid receptor activators cause appetite for fattier foods if they are injected into nucleus accumbens.

Opiates cause constipation and inhibited gag reflex but during withdrawals vomiting and defecating become much likelier. Possible that this is also side effect of old function FFAR had. If FFAR is throughout gastrointestinal tract then it could sense how much fatty food is left to absorb. If this receptor notices much fat left in digested food then it may relax intestinal smooth muscles so they wouldn`t push food out yet. If they also reduce release of mucus and gastric acids then intestines may become less slimy and nutrients may pass more easily. Relaxed intestine may also have more area to absorb through. Blocked gag reflex can motivate animals to keep on eating animals that could wiggle in their throat.

But as usual in body receptors adjust their amount and sensitivity to work in certain limits so long term opiate use may cause very active smooth muscles that force food out from any passage it could. Maybe this nausea started as warning mechanism that stops animals from eating too much fat.

THCV. Fats have usually long carbohydrate chain and THC happens to have long carbohydrate chain. While THC activates cannabinoid receptors THCV has bit shorter chain as only structural difference and this gives it almost opposite effects to THC (binds to receptors but doesn`t activate them). Looks like having cannabinoid receptors related to FFAR receptors makes them activate if they bind with something similar to fatty acid chain.

Anandamide above is produced by human body. It looks mostly like fat molecule but it activates cannabinoid receptors. Even artificial cannabinoids found in "spice" mixes have long (at least 4 carbon long) fatty chains. Shorter chains seem to bind weakly or just block cannabinoid receptors.